With the possible exception of darwin, all the major operating systems are fairly careful about maintaining backwards compatibility. They've certainly been making noises about cutting support for a long time, but it's not surprising that they were very deliberate about doing it for real.
There is a difference between ongoing support and security fixes support. Oracle database dropped support itanium on Linux since version 11. I suggest others did this as well. Of course companies wanted to extend their HW lifetime as much as possible and switch from everyday production use to some secondary functions. But having this power hungry monsters online is a waste of money on my mind.
It took extra long because for various reasons it was the officially blessed architecture (specifically, HP-UX on Itanium) for running Oracle databases, leading to HP suing Oracle when the latter bought Sun and essentially declared HP-UX is to soon be EOL'd